Seattle Desires To Streamline The ADU Process

Removing The Barriers For Attached Or Detached ADUs

Written By Janet Thome

In July, the Seattle City Council passed Council Bill 119544 for ADUs, taking away the barriers and making it easier for more property owners to build backyard cottages (detached accessory dwelling units or DADUs) and basement units (attached accessory dwelling units or AADUs) and therefore provide more housing options for people living in Seattle.

What Is An ADU?

Backyard Cottage ADU

An accessory dwelling unit (ADU) is a separate living space within a house or on the same property as an existing house. These units aren’t legal unless they have been established through a permit process. A legally permitted unit in the home is called an attached accessory dwelling unit (AADU). A legally permitted unit on the property (but not within the home) is called a detached accessory dwelling unit (DADU) or backyard cottage. Tiny houses, with foundations, are considered DADUs.

The Legislation:

  • Reduces the minimum lot size required to build a DADU on a single-family lot from 4,000 square feet to 3,200 square feet;
  • Increase the maximum size of DADUs from 800 square feet to 1,000 square feet, excluding any parking or storage areas;
  • Removes the owner-occupancy requirement for ADUs;
  • Removes the off-street parking requirement for ADUs;
  • Allows two ADUs on one lot (either one attached and one detached, or two attached) if the second ADU meets a green building standard or will be affordable to households at or below 80% of area median income;
  • Increases the maximum household size permitted on a single-family lot from 8 to 12 unrelated people only if the lot includes two ADUs;
  • Increases DADU height limits by 1-3 feet, with flexibility for green building strategies;
  • Allows design flexibility to preserve existing trees and to convert existing accessory structures to a DADU;
  • Require annual reporting on ADU production and requires that the Office of Planning and Community Development and the Seattle Department of Construction and Inspections conduct a survey of ADU owners and occupants within 3 years.
  • Introduces a Floor Area Limit (FAR) for all new development in single-family zones with some exemptions (this regulation has a delayed effective date until March 1, 2020);

Accessory Dwelling Unit Webpage

Effective March 1st, 2020

The regulations for accessory dwelling units were updated by Ordinance 125854 and the majority of the requirements are effective as of August 8, 2019. The updated requirements to floor area limits in single-family zones SF5000, SF7200, and SF9600 will be effective on March 1, 2020.

Executive Order To Encourage ADUs.

Mayor Jenny Durkan’s July 2019 Executive Order calls for other actions to encourage accessible and afford-able ADUs. In addition to pre-approved DADU plans, they  are developing an affordable ADU financing option, creating new online tools and resources, and monitoring ADU development annually.

To simplify and streamline permitting, the City is developing pre-approved DADU construction plans that offer a faster, easier, and more predictable design and permitting process.

Public Comments Welcome: Survey Open Until October 19th, 2019

Here’s how it works:1.The  public survey informs design principles and criteria we will use to select plans. 2. They  invite designers and builders to submit DADU designs.3. Permitting staff pre-approve 6-10 plans chosen based on selection criteria.4. Plans become available for homeowners, who can connect with the designer to create a site plan. Homeowners choosing a pre-approved DADU plan get a shorter permit review process and reduced permit fee.

Plans selected for pre-approval will be featured in an online gallery on the City’s ADU website.Why encourage ADUs? ADUs offer Seattle residents several opportunities: »More places to rent in neighborhoods where housing is often unaffordable»For owners, a path to generate income and wealth»Homes meeting the needs of families with children, aging in place, multi generational households, and people with disabilities.


The process to create an ADU can sometimes feel complex or intimidating. To simplify and streamline permitting, the City of Seattle is developing pre-approved construction plans for detached accessory dwelling units (DADUs), often called backyard cottages. Using a pre-approved DADU plan will provide a faster, easier, and more predictable design and permitting process.

This fall, we will ask the design community to submit DADU designs that the Seattle Department of Construction and Inspections (SDCI) will review and pre-approve. But first, we want to hear your ideas for design principles we should encourage through this process.

This survey takes about 15 minutes to complete. Your responses will inform how we evaluate the design submissions. Thank you for taking this time to support this process. To learn more, read our summary.

Take The Survey

ADU Fair: Resources For Creating Accessory Dwelling Units: October 19th, 2019

Want to learn more? On October 19, OPCD and other departments will be attending ADU Fair: Resources for creating accessory dwelling units, a free event at Southside Commons from 2:00 to 5:00 p.m. Come by to learn about ADUs, talk with staff, or even take the survey. You can also email  at, read our summary, and visit the SDCI website on ADUs.

Photo Credit: Clayton Homes


Scroll to Top